Cookware with copper bonded layer

ABSTRACT

Provided is an article of cookware and a method of making the same. The cookware has at least one stainless steel layer and at least one copper layer metallurgically bonded directly to the at least one stainless steel layer via solid state bonding. The at least one stainless steel layer may be a ferritic stainless steel layer, and the at least one copper layer may be a grain stabilized copper. The at least one stainless steel layer may be made from a 400 series stainless steel, such as a 436 stainless steel alloy, a 439 stainless steel alloy, or a 444 stainless steel alloy. The at least one copper layer may be made from a high purity, oxygen free copper alloy, such as a C101 copper alloy, a C102 copper alloy, or a C107 copper alloy.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 62/539,104, filed on Jul. 31, 2017, the disclosure of which ishereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to multi-ply, bonded cookware having atleast one copper layer bonded to at least one stainless steel layer. Amethod for making the bonded cookware using solid state bonding is alsodisclosed.

Description of Related Art

It has long been known to manufacture multi-layer bonded compositecookware in which various materials are joined together to combine thedesired physical properties of each of the materials into a composite.For example, the corrosion resistance of stainless steel is ideal forthe cooking surface as well as for the exterior surface of cookware;however, the thermal conductivity of stainless steel is not relativelyhigh. On the other hand, aluminum and/or copper offer much higherthermal conductivities and have been bonded to stainless steel toprovide well-known composite cookware items such as pots, pans,griddles, and the like. Multi-layer bonded cookware is known in the art,as shown in a number of patents, such as, for example: U.S. Pat. Nos.4,246,045 and 4,167,606 to Ulam; and U.S. Pat. Nos. 8,133,596 and6,267,830 to Groll. These patents demonstrate that the manufacture ofmulti-layer bonded cookware comprising stainless steel outer layersbonded to central layer(s) of a higher conductivity aluminum and/orcopper is well known in the art. The bonding between layers of thesedifferent materials is commonly achieved by conventional roll-bondingtechniques using strips of aluminum and/or copper, roll-bonded to outerstrips of stainless steel. It is known that roll-bonding between copper,aluminum, and stainless steel layers is conventional in the art ofmaking composite cookware and other food preparation apparatus.

A solid state bonding technique using high static pressure and heatapplied over time to make a plurality of composite blanks of, forexample, a combination of stainless steel-aluminum-stainless steel inthe manufactured cookware, is disclosed in U.S. Pat. No. 9,078,539 toGroll et al. There is a need in the art for producing cookware madeusing solid state bonding techniques for reducing the weight andimproving thermal characteristics of the cookware.

SUMMARY OF THE INVENTION

In view of the existing need in the art, it would be desirable todevelop new methods of producing cookware using solid state bondingtechniques. It would be further desirable to provide cookware made bysuch methods, wherein the cookware in some embodiments has reducedweight and improved thermal characteristics over existing cookware madeby solid state bonding techniques.

In accordance with one embodiment or aspect of the present disclosure,cookware may have a multi-layer, solid state bonded composite wallstructure. The cookware may have at least one stainless steel layer, andat least one copper layer metallurgically bonded to the at least onestainless steel layer via solid state bonding. The at least onestainless steel layer may be a ferritic stainless steel layer, and theat least one copper layer may be a grain stabilized copper.

In accordance with another embodiment or aspect of the presentdisclosure, the at least one stainless steel layer may be made from a300 series stainless steel or a 400 series stainless steel. The at leastone stainless steel layer may be made from a 436 stainless steel alloy,a 439 stainless steel alloy, or a 444 stainless steel alloy. The atleast one stainless steel layer may be made from a ferro-magneticstainless steel with chrome content of at least 17%. The grainstabilized copper may be selected from one of a C101 copper alloy, aC102 copper alloy, or a C107 copper alloy. The at least one stainlesssteel layer may have a thickness between about 0.010 inches to about0.100 inches, more preferably 0.015 inches to about 0.025 inches. The atleast one copper layer may have a thickness between about 0.010 inchesto about 0.25 inches, more preferably 0.05 inches to about 0.150 inches.The at least one stainless steel layer may be circular with a diameterbetween about 5 inches to about 25 inches. The at least one copper layermay be circular with a diameter between about 5 inches to about 25inches. The at least one stainless steel layer and the at least onecopper layer may be circular, and wherein a diameter of the at least onestainless steel layer may be equal to or larger than a diameter of theat least one copper layer. The at least one stainless steel layer andthe at least one copper layer may be circular, and wherein a center ofthe at least one stainless steel layer may be on a common axis with acenter of the at least one copper layer. The cookware may be formed as afrying pan. Of course, other geometric shapes may be used such assquare, rectangular, oval, and the like may be used if desired dependingupon the final desired shape of the cookware or cook surface.

In accordance with another embodiment or aspect of the presentdisclosure, cookware may have a multi-layer, solid state bondedcomposite wall structure. The cookware may have an upper stainless steellayer and a lower stainless steel layer, and a copper layer between theupper stainless steel layer and the lower stainless steel layer. Thecopper layer may be metallurgically bonded to the upper stainless steellayer and the lower stainless steel layer via solid state bonding. Theupper stainless steel layer and the lower stainless steel layer may bemade from a ferritic stainless steel, and the copper layer may be madefrom a copper alloy comprising silver. A ring-shaped portion of thelower stainless steel layer may be removed around a perimeter of thecookware to visually expose a portion of the copper layer.

In accordance with a still further embodiment or aspect of the presentdisclosure, cookware may have a multi-layer, solid state bondedcomposite wall structure. The cookware may have an upper stainless steellayer defining the cook surface, a copper layer, a lower stainless steellayer, and a copper layer forming the exterior, outer surface of thecookware. Hence, a four-layer structure is provided by this embodiment.

Still further, another embodiment of the present disclosure, cookwaremay have a multi-layer, solid state bonded composite wall structurecomprising a five-layer composite having an upper stainless steel layerdefining the cook surface, a copper layer, a stainless steel layer, acopper layer, and a stainless steel layer forming the exterior outersurface of the cookware.

In accordance with another embodiment or aspect of the presentdisclosure, a method of making multi-layer, bonded cookware may includeproviding at least one stainless steel layer and at least one copperlayer in a stacked blank assembly, and applying heat and pressure to thestacked blank assembly for a predetermined period of time such that atleast one stainless steel layer is metallurgically bonded to the atleast one copper layer via solid state bonding. The at least onestainless steel layer may be a ferritic stainless steel layer, and theat least one copper layer may be a grain stabilized copper.

In accordance with another embodiment or aspect of the method of thepresent disclosure, heat may be applied at a temperature below a graingrowth temperature of the at least one copper layer. Heat may be appliedat a temperature between about 625° C. to about 675° C. (1150° F. to1250° F.). Pressure may be applied at about 5,000 psi to about 20,000psi (350 kg/cm² to 1,400 kg/cm²). Pressure may be applied in a directionnormal to a plane of the at least one stainless steel plate and the atleast one copper plate while the stacked plates are at elevatedtemperature so as to achieve solid state bonding between the plates. Thepredetermined period of time may be about 1 hour to about 3 hours. Thestep of applying heat and pressure may be carried out by an inductionheating coil surrounding the blank assembly. A non-oxidizing atmospheremay be present between the induction heating coil and the blankassembly. The method may further include forming the bonded blankassembly into a frying pan shape using a drawing press or a hydroformmachine, or the like.

These and other features and characteristics of the cookware describedherein, as well as methods of making such cookware, will become moreapparent upon consideration of the following description and theappended claims with reference to the accompanying drawings, all ofwhich form a part of this specification, wherein like reference numeralsdesignate corresponding parts in the various figures. It is to beexpressly understood, however, that the drawings are for the purpose ofillustration and description only.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is an exploded, side cross-sectional view of a blank assembly ofone embodiment or aspect of the present disclosure;

FIG. 2 is a cross-sectional view of a bonded blank assembly of FIG. 1;

FIG. 3 is an exploded side view of a blank assembly of anotherembodiment or aspect of the present disclosure;

FIG. 4 is a cross-sectional view of a bonded blank assembly of FIG. 3;and

FIG. 5 is a cross-sectional view of a formed fry pan made from thebonded blank assembly of FIG. 4;

FIG. 6 is an enlarged, partially fragmented, cross-sectional view of afour layer, bonded composite of one embodiment of the present invention;

FIG. 7 is a view similar to FIG. 6 but of a five layer bonded compositeof another embodiment of the invention;

FIG. 8 is a partially fragmented, cross-sectional view of a fry pan madefrom the five layer bonded composite of FIG. 7 with a skived ring formedaround the circumference thereof;

FIG. 9 is an enlarged cross-sectional view of the sidewall and skivedgroove taken a section circle IX of FIG. 8; and

FIG. 10 is a side-elevational view of a fry pan made from the four-layercomposite of FIG. 6 according to a still further embodiment of theinvention where the bottom, exterior layer of copper is removed toexpose the stainless steel bottom layer.

In FIGS. 1-10, the same characters represent the same components unlessotherwise indicated.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the singular form of “a”, “an”, and “the” includesplural referents unless the context clearly dictates otherwise.

As used herein, spatial or directional terms, such as “left”, “right”,“up”, “down”, “inner”, “outer”, “above”, “below”, and the like, relateto various features as depicted in the drawing figures. However, it isto be understood that various alternative orientations can be assumedand, accordingly, such terms are not to be considered as limiting.

Unless otherwise indicated, all ranges or ratios disclosed herein are tobe understood to encompass any and all subranges or subratios subsumedtherein. For example, a stated range or ratio of “1 to 10” should beconsidered to include any and all subranges between (and inclusive of)the minimum value of 1 and the maximum value of 10; that is, allsubranges or subratios beginning with a minimum value of 1 or more andending with a maximum value of 10 or less, such as but not limited to, 1to 6.1, 3.5 to 7.8, and 5.5 to 10.

As used herein, the term “substantially parallel” means a relative angleas between two objects (if extended to theoretical intersection), suchas elongated objects and including reference lines, that is from 0° to5°, or from 0° to 3°, or from 0° to 2°, or from 0° to 1°, or from 0° to0.5°, or from 0° to 0.25°, or from 0° to 0.1°, inclusive of the recitedvalues.

All documents, such as but not limited to issued patents and patentapplications, referred to herein, and unless otherwise indicated, are tobe considered to be “incorporated by reference” in their entirety.

As used herein, the term “solid state bonding” means a method of bondingtwo or more stacked plates of metals or metal alloys together using highstatic pressure (typically over 5,000 psi (350 kg/cm²)) and hightemperature (typically over 600° F. (315° C.), wherein the high staticpressure is applied in a normal or perpendicular direction, i.e., 90°relative to the plane of the stacked plates.

As used herein, the term “metallurgical bonding” or “metallurgicallybonded” refers to a bond formed between similar or dissimilar materialsthat is free of voids or discontinuities.

As used herein, the term “grain stabilized copper” refers to any highpurity, deoxidized copper or copper alloy having some residual hardnessor temper from fully annealed up to ½ hard that exhibits controlledgrain growth properties during solid state bonding.

With reference to the drawings, FIGS. 1-2 depict various views of ablank assembly 2 used in making one presently preferred and non-limitingembodiment of the cookware of the present invention. After appropriatesurface preparation of the various layers of materials to be used in abonding step, the materials are positioned in an ordered array to createthe blank assembly 2 as shown. In some examples, the surface preparationsteps may include degreasing, surface abrasion by chemical or mechanicalmethods, and the like.

The blank assembly 2 has upper and lower layers or plates 4 and 8 whichwill form the inner and outer surfaces, respectively, of the cookwareafter the bonding and forming steps. Each of the upper and lower plates4 and 8 has a top or upper surface (4 a, 8 a) and a bottom or lowersurface (4 b, 8 b). The upper and lower plates 4 and 8 may be discsabout 14 inches (355 mm) in diameter to form a near-net size blank formaking, for example, a fry pan of 10 inches (254 mm) in diameter. Inother examples, the upper and lower plates 4 and 8 may be discs fromabout 5 inches (127 mm) to about 25 inches (635 mm) in diameter. One ofordinary skill in the art would readily appreciate that the size of theupper and lower plates 4 and 8 can be increased or decreased to make frypans of larger or smaller sizes, respectively. While FIGS. 1-2 showupper and lower plate 4 and 8 having identical diameters, one ofordinary skill in the art would readily appreciate that a diameter ofthe upper plate 4 may be larger or smaller than a diameter of the lowerplate 8. In some examples, the diameter of the upper plate 4 may beabout 5 inches (127 mm) to 25 inches (635 mm). In some examples, thediameter of the lower plate 8 may be about 5 inches (127 mm) to 25inches (635 mm). The thicknesses of individual plates may be adjusted toachieve desired product weight and thermal performance.

In one exemplary and non-limiting embodiment, one of the upper and lowerplates 4 and 8 is formed from stainless steel, while the other of theupper and lower plates 4 and 8 is formed from copper, as discussedherein. For example, the upper plate 4 may be made from stainless steel,while the bottom plate 8 may be made from copper. With reference to FIG.2, the bottom surface 4 b of the upper plate 4 is positioned over thetop surface 8 a of the lower plate 8 and the two plates aremetalurgically bonded together using a solid state bonding technique tobe in intimate, thermally-conductive contact with one another, asdescribed herein.

With reference to FIGS. 3-5, a blank assembly 2′ is shown in accordancewith another preferred and non-limiting embodiment. The blank assembly2′ may be used in making cookware. After appropriate surfacepreparation, the materials constituting the blank assembly 2′ arepositioned in an ordered array. In some examples, the surfacepreparation steps may include degreasing, surface abrasion by chemicalor mechanical methods, and the like.

The blank assembly 2′ has an intermediate plate 6′ positioned between anupper plate 4′ and a lower plate 8′. The upper and lower plates 4′ and 8form the inner and outer surfaces, respectively, of the cookware afterthe solid state bonding and forming steps. Each of the upper and lowerplates 4′ and 8′ has a top or upper surface (4 a′, 8 a′) and a bottom orlower surface (4 b′, 8 b′). Similarly, the intermediate plate 6′ has atop or upper surface 6 a′ and a bottom or lower surface 6 b′. Withreference to FIG. 3, the bottom surface 4 b′ of the upper plate 4′ ispositioned over the top surface 6 a′ of the intermediate plate 6′, whilethe bottom surface 6 b′ of the intermediate plate 6′ is positioned overthe top surface 8 a′ of the lower plate 8′. The three plates aremetalurgically bonded together, such as shown in FIG. 4, using a solidstate bonding technique to be in intimate, thermally-conductive contactwith one another, as described herein.

The upper plate 4′, the intermediate plate 6′, and the lower plate 8′may be discs about 14 inches (355 mm) in diameter to form a near-netsize blank for making, for example, a fry pan of 10 inches (254 mm) indiameter. In some examples, a diameter of the intermediate plate 6′ maybe smaller than diameters of the upper and lower plates 4′ and 8′. Inother examples, the upper plate 4′, the intermediate plate 6′, and thelower plate 8′ may be discs from about 5 inches (127 mm) to about 25inches (635 mm) in diameter. One of ordinary skill in the art wouldreadily appreciate that the size of the upper plate 4′, the intermediateplate 6′, and the lower plate 8′ can be increased or decreased to makefry pans of larger or smaller sizes, respectively. While FIGS. 3-4 showthe upper plate 4′, the intermediate plate 6′, and the lower plate 8′having identical diameters, one of ordinary skill in the art wouldreadily appreciate that the diameters of upper plate 4′, theintermediate plate 6′, and the lower plate 8′ may be different from oneanother. In some examples, the thickness of the upper plate 4′ may beabout 0.010 inches (0.254 mm) to 0.10 inches (2.54 mm), the thickness ofthe intermediate plate 6′ may be about 0.010 inches (0.254 mm) to 0.250inches (6.35 mm), more preferably about 0.05 inches (1.27 mm) to about0.150 inches (3.81 mm), and the thickness of the lower plate 8′ may beabout 0.010 inches (0.254 mm) to 0.100 inches (2.54 mm). The thicknessesof individual plates may be adjusted to achieve desired product weightand thermal performance. The upper and lower plates 4′ and 8′ may beformed from stainless steel, while the intermediate plate 6′ may beformed from copper, and the three plates are metalurgically bondedtogether using a solid state bonding technique, as described herein. Thestainless steel plates may be replaced by titanium in other embodiments.When the lower plate 8′ is a ferritic grade of stainless steel intendedfor use in induction heating, the preferred thickness of the ferriticstainless steel lower plate 8′ is about 0.015 inches (0.381 mm) to about0.025 inches (0.635 mm) so as to improve the heating efficiency byincreasing the inductive effect.

While FIGS. 1-4 show preferred and non-limiting examples of blankassemblies 2, 2′ made using a stacked arrangement of two plates (FIGS.1-2) or three plates (FIGS. 3-4), other arrangements comprising aplurality of plates may also be formed. For example, a four or fiveplate blank assembly may be made by alternately stacking stainless steeland copper plates. After solid state bonding, the four plate blankassembly 40, partially depicted in FIG. 6, has a first stainless steelplate 42 on the top cook surface bonded to a copper plate 44, which isbonded to a second stainless steel plate 46 which, in turn, is bonded toa second copper plate 48 which forms an exterior surface of the formedcookware. When formed, the five plate blank assembly 50, partially shownin FIG. 7, comprises stainless steel plates 52, 52′ on top and bottomsurfaces with a copper-stainless steel-copper intermediate layer betweenthe top and bottom stainless steel plates 52, 52′.

In the four and five layer embodiments depicted in FIGS. 6 and 7, havinginternal layers 46 and 56, respectively, of stainless steel, the formedcookware will exhibit a more uniform heat distribution on the cooksurface because the internal layers of stainless steel acts as a thermalbarrier so as to spread the heat radially. This concept of using aso-called “heat dam” of stainless steel in a core layer of cookware isdisclosed in our prior patents, namely, U.S. Pat. Nos. 6,926,971;7,208,231; 7,906,221; and 8,133,596, which are incorporated by referenceherein. These prior patents, however, are based on a roll-bonded methodof forming the bonded composite and require a layer of pure aluminum orAlclad aluminum bonded to both sides of the stainless steel layer inorder to achieve a metallurgical bond therewith in roll-bonding thevarious metal layers of the composite. We have discovered that the useof the aluminum or Alclad layers are not necessary when using the solidstate bonding technique of the present invention.

The three layer bonded blank assembly 2′ of FIG. 4 may be formed in theshape of cookware, such as a frying pan 10 depicted in FIG. 5. Thebonded blank assembly 2′ may be drawn, such as by using a press, intothe shape of the frying pan 10 having a cooking surface 12 and acircumferential sidewall 14 surrounding the cooking surface 12. Thesidewall 14 may have a radiused lower edge 16 at a transition with thecooking surface 12. A lip 18 may be formed at an upper end of thesidewall 14 opposite the radiused edge 16. As shown in FIG. 5, theintermediate layer 6′, may be bonded with the upper layer 4′ and thelower layer 8′ over an entire cross-sectional area of the frying pan 10.In some examples, such as when the intermediate layer 6′ has a smallerdiameter compared to the upper layer 4′ and the lower layer 8′, theintermediate layer 6′ may be bonded with the upper layer 4′ and thelower layer 8′ in a first portion of the frying pan 10, while the upperand lower layers 4′ and 8′ may be bonded directly to one another in asecond portion of the frying pan 10. For example, the intermediate layer6′ may be provided only in the area of the cooking surface 12 of thefrying pan 10, while the upper and lower layers 4′ and 8′ may be bondeddirectly to one another in the area of the sidewall 14 of the frying pan10.

With reference to FIGS. 7-9, a skived ring 55 can be formed around thefive-layer cookware 50′ by machining away an outer portion of thestainless steel layer 52′ to expose the underlying layer of copper 58,to not only provide a pleasing visual copper ring effect but also toprovide a quick identification of the type of cookware selected by theuser. Such a skived ring may also be formed on the three-layerembodiment of cookware of FIG. 5 (not shown). Other decorativetechniques may be employed by selectively removing portions of the outerlayer 52′ of stainless steel in various patterns, such as by lasercutting, to visually expose the underlying copper layer 58.

Another embodiment is depicted in FIG. 10 which shows a fry-pan 60 madefrom the four layer construction of the blank assembly 40 shown in FIG.6. In this embodiment, the lowermost layer 70 of copper 48 is removedafter the forming of the fry pan, as by machining, to expose thelowermost layer 46 of stainless steel. In this embodiment, the exposedsurface of the ferritic stainless steel makes the cookware compatiblefor induction heating at the bottom, while providing a sidewall surfaceof copper 48.

Having described the structure of the blank assembly in accordance withvarious embodiments or aspects of the present disclosure, a method ofmaking cookware, such as the frying pan 10, using the blank assemblywill now be described. The solid state bonding technique of bondingpre-cut near net shape plate blanks not only reduces scrap lossesheretofore encountered in the conventional roll bonding manufacture ofcomposite cookware but also permits the use of other materials in makingmultiple composites which have proven difficult, impossible and/orexpensive to roll-bond. For example, solid state bonding permits the useof different grades of stainless steel than otherwise possible inconventional roll bonding so as to lower costs of materials. The desiredresult after the bonding is to have a multi-layered disc that hassufficient bond strength to withstand the stress of forming, fine grainstructure in the copper to avoid extreme rough texture of the formedpart and corrosion resistance which is suitable for a food preparationsurface.

Initially, the blank assembly 2 is formed by stacking upper plate 4 andthe lower plate 8 such that the lower surface 4 b of the upper plate 4is on top of the upper surface 8 a of the lower plate 8. In the case ofthe blank assembly 2′ shown in FIGS. 3-4, In the case of the blankassembly 2 shown in FIGS. 1-3, the upper plate 4′, the intermediateplate 6′, and the lower plate 8′ are stacked such that the bottomsurface 4 b′ of the upper plate 4′ is positioned over the top surface 6a′ of the intermediate plate 6′, while the bottom surface 6 b′ of theintermediate plate 6′ is positioned over the top surface 8 a′ of thelower plate 8′. Desirably, the plates are aligned such that the centersof each plate share a common axis. In some examples, the plates may bestacked such that their centers are offset from one another. Whenstacked, the upper plate 4 and the lower plate 8 (FIG. 2), or the upperplate 4′, the intermediate plate 6′, and the lower plate 8′ aresubstantially parallel to each other. It will be understood that thissame or similar stacking technique is employed when making theembodiments of FIGS. 6 and 7.

The blank assembly is then placed in a press apparatus (not shown) forapplication of a load or pressure in a direction normal (i.e.,perpendicular) relative to the planes of plates in the blank assembly.Multiple blank assemblies may be produced in the same press cycle bystacking blank assemblies and placing a high temp separation materialbetween the stacked blank assemblies that are not intended to bond.Pressure is applied evenly across the surface of the plates to expel airfrom the stacked blank assembly and prevents air encroachment during thebonding cycle. A protective atmosphere different from the surroundingatmosphere may also be introduced around the blank assembly to preventencroachment of the surrounding atmosphere between the plates of theblank assembly. The protective atmosphere may be a non-oxidizingatmosphere. Without intending to be bound by theory, it has been foundthat the protective atmosphere prevents encroachment of ambient airbetween the plates of the blank assembly during the solid state bondingprocess, thereby allowing for a reduction in pressure necessary toachieve a strong bond between the plates of the blank assembly. Whileunder a pressure of between about 5,000 psi and 20,000 psi (350 kg/cm²to 1,400 kg/cm²), heat is applied to the blank assembly or assemblies 2between about 800° F. and 1,400° F. (427° C. to 760° C.) for asufficient time (about 1-3 hours) to achieve solid state bonding (i.e.,metallurgical bonding) between the plates in the blank assembly orassemblies. In one example, the stacked blank assembly is quickly heatedunder pressure to a temperature of 1150° F. +/−75° F. (620° C. +/−25°C.), such as using an induction heating device (i.e., at least oneinduction heating coil) which surrounds the stacked blank assemblies.

Each bonded blank assembly is then removed from the press apparatus andallowed to cool. In some examples, cooling may be accomplished byexposure to ambient air or by using a cooling agent, such as forced airor liquid.

After solid state bonding, the bonded blank assembly is formed in adrawing press or hydroform machine (not shown) into a desired shape,such as a frying pan shape 10 depicted in FIG. 5. A handle or handles(not shown) may be attached to the cookware in a known manner.

In various examples, the blank assembly may have at least one plate madefrom copper. While copper is typically used in cookware for its highthermal conductivity, various parameters of the solid state bondingprocess must be controlled to prevent undesirable grain growth incopper. The presence of heat, pressure and holding time all contributeto the quality of the bond between the layers of plates. A temperaturehigher than 1,250° F. (675° C.) and a holding time longer than 3 hoursgenerally result in a higher bond strength. However, the higher heat andlonger hold time result in undesirable grain growth in copper. A metalsuch as copper has a grain structure that can range from very coarse tovery fine and is highly influenced by the chemistry of the metal and theamount of cold work that the metal has undergone. Without intending tobe bound by the theory of grain growth, the same process (time,temperature, pressure) that promotes bond strength, can also promotegrain growth in a copper plate. For example, further increasing thetemperature to 1,380° F. (750° C.) results in a blank assembly thatexhibits extreme texture in the areas of the formed cookware thatundergo the most deformation during forming. This texture is difficultor impossible to polish, and also weakens the bonded assembly, therebymaking it susceptible to breakage during forming.

In order to control the grain growth in copper due to exposure totemperature higher than 1,250° F. (675° C.), an alloy of copper, such asa copper alloy containing iron, may be used. The addition of ironstabilizes the grain structure of copper at elevated temperature (higherthan 1,250° F. (675° C.)). However, the addition of iron greatlydecreases the conductivity of the copper alloy compared to high puritycopper (35%). For example, high purity copper has thermal a conductivityof 388 W/mK, while alloyed copper having 2% iron has a thermalconductivity of 260 W/mK. For comparison, pure aluminum has a thermalconductivity of 222 W/mK. Copper is typically used in cookware for itshigh thermal conductivity. Thus, addition of grain-stabilizing iron tohigh purity copper is undesirable because it reduces its thermalconductivity by 35% to a level that is similar to that of pure aluminum.Even though iron stabilized copper may not be a preferred copper alloyfrom the standpoint of thermal conductivity, this alloy may still beused.

Adding silver to high purity copper has been found to promote grainstability at elevated temperatures. For example, adding pure silver tohigh purity, deoxidized copper at a concentration of 0.8 kg/ton (0.0034wt. % Ag), such as in the C107 copper alloy, sold by Hussey Copper,increases the grain size stability without negatively affecting thethermal conductivity of the resulting alloy compared to unalloyed highpurity, deoxidized copper. Grain growth can be further controlled byproviding a high purity, deoxidized copper plate in a fully or partiallyannealed condition, allowing some residual hardness, for example up to ½hard in copper alloys, such as C101, C102, and C107 oxygen free copperalloys. In some examples, C103, C104, and C105 oxygen free copper alloysmay also be used.

Care should be taken in choosing the proper type of C107 copper alloy soas to insure that it does not contain any alloy additions that may beharmful for use in food preparation items such as cookware. For example,it will be noted that at least one brand of C107 copper alloy marketedby Columbia metals, Ltd. (UK) contains 0.35 wt. % arsenic, which may notbe suitable for use in food preparation goods.

In addition to a copper plate, the blank assembly 2 or 2′ may have atleast one plate made from stainless steel. The stainless steel may bemade of a ferro-magnetic (ferritic) stainless steel in order to make thefinished cookware suitable for use on an induction cooking apparatus. Insome examples, titanium or titanium alloys may be substituted for one ormore of the stainless steel plates.

The austenitic (nickel bearing) grades of stainless steel have long beenthe standard of the food preparation industry. These grades, however,are subject to intergranular corrosion after being subject totemperatures between 1,000° F. to 1,650° F. (540° C. to 900° C.). Tore-establish corrosion resistance, the material must be heated to atemperature between 1,650° F. to 2,050° F. (900° C. to 1,120° C.) andrapidly quenched to room temperature. However, this temperature is inthe grain growth range and near or above the melting temperature ofcopper. Thus, austenitic grades of stainless steel are impractical forforming a blank assembly 2 or 2′ using a solid state bonding technique.

To overcome the intergranular corrosion problems of austenitic stainlesssteel alloys at bonding temperatures during the solid state bondingprocess, ferritic stainless steel alloys, such as the 436, 439, 444, andchrome shield 22 alloys, may be used. These ferritic alloys containelements such as copper, titanium, and niobium that bond with carbon toprevent the formation of chrome carbides. In some examples, the ferriticstainless steel alloy may be low carbon, grain-stabilized, ferriticstainless steel with chrome content of at least 17 wt. %. In the case ofa two-sided stainless clad plate, a ferritic grade stainless steel isdesirably used on both sides to promote flatness and stability fordrawing and making the cookware induction-capable.

Table 1 below summarizes various process parameters and materials usedin a solid state bonding process for making a blank assembly suitablefor being formed into cookware. Parameters marked with an “X” designatean undesirable process variable or material. Parameters marked with an“O” do not have an effect on process variable or material. Parametersmarked with a “✓” designate a desirable process variable or material.

TABLE 1 Corro- sion Con- Bond Grain Resis- duc- Shape Parameter StrengthGrowth tance tivity Stability Temp. below 625° C. X ✓ ◯ ◯ ◯ Temp. 650°C. (+/− 25° C.) ✓ ✓ ◯ ◯ ◯ Temp. above 675° C. ✓ X ◯ ◯ ◯ Temp. holdlonger than 3 h ✓ X ◯ ◯ ◯ Temp. hold shorter than 1 h X ✓ ◯ ◯ ◯ Temp.hold 1-3 h ✓ ✓ ◯ ◯ ◯ Austenitic stainless ◯ ◯ X ◯ X (one side) Ferriticstainless (two sides) ◯ ◯ ✓ ◯ ✓ Copper-iron alloy ✓ ✓ ◯ X ◯Copper-silver alloy - fully ✓ ✓ ◯ ✓ ◯ annealed up to ½ hard

Below are listed some metal combinations that can be made by thetechnique described above. The thicknesses of individual layers may beadjusted to achieve desired product weight and thermal performance.Repeated layers of like metals need not be of the same thickness.

-   Stainless/Copper-   Stainless/Copper/Stainless-   Stainless/Copper/Stainless/Copper-   Stainless/Copper/Stainless/Copper/Stainless-   Multiple bank assemblies may be produced in the same press cycle by    stacking assemblies and placing a high temperature separation    material between assemblies that are not intended to bond, as    mentioned herein above. The above layer combinations do not limit    the number of repetitions in forming the blank assemblies with    additional layers of stainless steel and copper as desired.

Copper is used in cookware for its high thermal conductivity. As can beseen above, the conductivity of the iron-copper alloy is only 65% ofpure copper and is not much different than the conductivity of purealuminum. Copper adds weight and expense to cookware. We recommend theuse of copper with the highest possible conductivity. The copper alloywe have arrived at for cookware products is high purity, deoxidizedcopper that has a small addition of pure silver at a concentration of0.8 kg/metric ton. The silver addition gives an increase in grain sizestability to a higher (+122° F.; +50° C.) temperature than copper alloywithout silver. The silver has no negative effect on the conductivity ofthe copper alloy. The copper alloy is known in the industry as C107. Itis a deoxidized grade and is the material recommended as part of thisdisclosure. While C107 copper alloy performs very well, we have foundgenerally that high purity, oxygen free copper alloys having someresidual hardness or temper from fully annealed up to ½ hard haveexhibited controlled grain growth properties during solid state bondingat elevated temperatures, including C107, C101, and C102 oxygen-freecopper alloys.

In various examples, the present invention may be further characterizedby one or more of the following clauses:

Clause 1. Cookware having a multi-layer, solid state bonded compositewall structure, the cookware comprising:

at least one stainless steel layer; and

at least one copper layer metallurgically bonded to the at least onestainless steel layer via solid state bonding, and

wherein the at least one copper layer is a grain stabilized copper.

Clause 2. The cookware of clause 1, wherein the at least one stainlesssteel layer is made from a 300 series stainless steel or a 400 seriesstainless steel.

Clause 3. The cookware of clause 1 or clause 2, wherein the at least onestainless steel layer is made from a 436 stainless steel alloy, a 439stainless steel alloy, or a 444 stainless steel alloy.

Clause 4. The cookware of any of clauses 1-3, wherein the at least onestainless steel layer is made from a ferro-magnetic stainless steel withchrome content of at least 17%.

Clause 5. The cookware of any of clauses 1-4, wherein the grainstabilized copper is one selected from a C101 copper alloy, a C102copper alloy, and a C107 copper alloy.

Clause 6. The cookware of any of clauses 1-5, wherein the at least onestainless steel layer has a thickness between about 0.01 inches (0.254mm) to about 0.10 inches (2.54 mm).

Clause 7. The cookware of any of clauses 1-6, wherein the at least onecopper layer has a thickness between about 0.01 inches (0.254 mm) toabout 0.25 inches (6.35 mm).

Clause 8. The cookware of any of clauses 1-7, wherein the at least onestainless steel layer is circular with a diameter between about 5 inches(127 mm) to about 25 inches (635 mm).

Clause 9. The cookware of any of clauses 1-8, wherein the at least onecopper layer is circular with a diameter between about 5 inches (127 mm)to about 25 inches (635 mm).

Clause 10. The cookware of any of clauses 1-9, wherein the at least onestainless steel layer and the at least one copper layer are circular,and wherein a diameter of the at least one stainless steel layer isequal to or larger than a diameter of the at least one copper layer.

Clause 11. The cookware of any of clauses 1-10, wherein the at least onestainless steel layer and the at least one copper layer are circular,and wherein a center of the at least one stainless steel layer is on acommon axis with a center of the at least one copper layer.

Clause 12. The cookware of any of clauses 1-11, wherein the cookware isformed as a frying pan.

Clause 13. Cookware having a three-layer, bonded composite wallstructure, the cookware comprising: an upper stainless steel layer and alower stainless steel layer; and a copper layer between the upperstainless steel layer and the lower stainless steel layer, the copperlayer metallurgically bonded directly to the upper stainless steel layerand the lower stainless steel layer.

Clause 14. The cookware of clause 13, wherein a portion of the lowerstainless steel layer is removed, such as by a skived ring or a laserinscribed pattern, to visually expose an underlying surface of thecopper layer.

Clause 15. The cookware of clause 13 or clause 14, wherein the upperstainless steel layer and the lower stainless steel layer are made froma ferritic stainless steel, and wherein the copper layer is made from agrain stabilized copper.

Clause 15a. The cookware of clauses 15, wherein the grain stabilizedcopper is one selected from a C101 copper alloy, a C102 copper alloy,and a C107 copper alloy.

Clause 16. Cookware having a four-layer, bonded composite wallstructure, the cookware comprising:

a first layer of stainless steel defining a cook surface of thecookware;

a first layer of copper directly bonded to the first layer of stainlesssteel;

a second layer of stainless steel directly bonded to the first layer ofcopper;

a second layer of copper directly bonded to the second layer ofstainless steel, said second layer of copper defining an exteriorsurface of the cookware.

Clause 17. The cookware of clause 16, wherein a lowermost surface of thesecond layer of copper is removed to expose an underlying surface of thesecond layer of stainless steel, whereby the cookware is compatible withinduction heating, while an outer sidewall of the cookware is defined bythe second layer of copper.

Clause 18. The cookware of clause 16 or clause 17, wherein the upperstainless steel layer and the lower stainless steel layer are made froma ferritic stainless steel, and wherein the copper layer is made from agrain stabilized copper.

Clause 18a. The cookware of clause 18, wherein the grain stabilizedcopper is one selected from a C101 copper alloy, a C102 copper alloy,and a C107 copper alloy.

Clause 19. Cookware having a five-layer bonded composite wall structure,the cookware comprising:

a first layer of stainless steel defining a cook surface of thecookware;

a first layer of copper directly bonded to the first layer of stainlesssteel;

a second layer of stainless steel directly bonded to the first layer ofcopper;

a second layer of copper directly bonded to the second layer ofstainless steel;

a third layer of stainless steel directly bonded to the second layer ofcopper, defining an exterior surface of the cookware.

Clause 20. The cookware of clause 19, wherein a portion of the thirdlayer of stainless steel is removed to visually expose an underlyingsurface of the second layer of copper.

Clause 21. The cookware of clause 20, wherein the visually exposedportion is one of a skived ring or a laser inscribed pattern.

Clause 22. The cookware of any of clauses 19-21, wherein the first,second, and third layers of stainless steel are made from a ferriticstainless steel and wherein the first and second layers of copper aremade from a grain stabilized copper.

Clause 23. The cookware of clause 22, wherein the grain stabilizedcopper is one selected from a C101 copper alloy, a C102 copper alloy,and a C107 copper alloy.

Clause 24. A method of making multi-layer, bonded cookware, the methodcomprising:

providing at least one stainless steel layer and at least one copperlayer in a stacked blank assembly; and

applying heat and pressure to the stacked blank assembly for apredetermined period of time such that at least one stainless steellayer is metallurgically bonded to the at least one copper layer viasolid state bonding,

wherein the at least one stainless steel layer is a ferritic stainlesssteel layer, and

wherein the at least one copper layer is a grain stabilized copper.

Clause 25. The method of clause 24, wherein heat is applied at atemperature below a grain growth temperature of the at least one copperlayer.

Clause 26. The method of clause 24 or clause 25, wherein heat is appliedat a temperature between about 1150° F. (625° C.) to about 1250° F.(675° C.).

Clause 27. The method of any of clauses 24-26, wherein pressure isapplied at about 5,000 psi (350 kg/cm²) to about 20,000 psi (1,400kg/cm²).

Clause 28. The method of any of clauses 24-27, wherein pressure isapplied in a direction normal to a plane of the at least one stainlesssteel plate and the at least one copper plate.

Clause 29. The method of any of clauses 24-28, wherein the predeterminedperiod of time is about 1 hour to about 3 hours.

Clause 30. The method of any of clauses 24-29, wherein the step ofapplying heat and pressure is carried out by an induction heating coilsurrounding the blank assembly and wherein a non-oxidizing atmosphere ispresent between the induction heating coil and the blank assembly.

Clause 31. The method of any of clauses 24-30, further comprisingforming the bonded blank assembly into a frying pan shape using adrawing press or a hydroform machine.

The present invention has been described with reference to specificdetails of particular examples thereof. It is not intended that suchdetails be regarded as limitations upon the scope of the inventionexcept insofar as and to the extent that they are included in theaccompanying claims.

The invention claimed is:
 1. Cookware having a four-layer, bondedcomposite wall structure, the cookware comprising: a first layer ofstainless steel defining a cook surface of the cookware; a first layerof copper directly bonded to the first layer of stainless steel; asecond layer of stainless steel directly bonded to the first layer ofcopper; a second layer of copper directly bonded to the second layer ofstainless steel, said second layer of copper defining an exteriorsurface of the cookware.
 2. The cookware of claim 1, wherein a lowermostsurface of the second layer of copper is removed to expose an underlyingsurface of the second layer of stainless steel, whereby the cookware iscompatible with induction heating, while an outer sidewall of thecookware is defined by the second layer of copper.
 3. The cookware ofclaim 2, wherein the visually exposed portion is one of a skived ring ora laser inscribed pattern.
 4. The cookware of claim 1, wherein the firstand second layers of stainless steel are made from a ferritic stainlesssteel and wherein the first and second layers of copper are made from agrain stabilized copper.
 5. The cookware of claim 4, wherein the grainstabilized copper is a C107 copper alloy comprising silver.
 6. Thecookware of claim 4, wherein the grain stabilized copper is a C101copper alloy.
 7. The cookware of claim 4, wherein the grain stabilizedcopper is a C102 copper alloy.
 8. Cookware having a five-layer bondedcomposite wall structure, the cookware comprising: a first layer ofstainless steel defining a cook surface of the cookware; a first layerof copper directly bonded to the first layer of stainless steel; asecond layer of stainless steel directly bonded to the first layer ofcopper; a second layer of copper directly bonded to the second layer ofstainless steel; a third layer of stainless steel directly bonded to thesecond layer of copper, defining an exterior surface of the cookware. 9.The cookware of claim 8, wherein a portion of the third layer ofstainless steel is removed to visually expose an underlying surface ofthe second layer of copper.
 10. The cookware of claim 9, wherein thevisually exposed portion is one of a skived ring or a laser inscribedpattern.
 11. The cookware of claim 8, wherein the first, second, andthird layers of stainless steel are made from a ferritic stainless steeland wherein the first and second layers of copper are made from a grainstabilized copper.
 12. The cookware of claim 11, wherein the grainstabilized copper is a C107 copper alloy comprising silver.
 13. Thecookware of claim 11, wherein the grain stabilized copper is a C101copper alloy.
 14. The cookware of claim 11, wherein the grain stabilizedcopper is a C102 copper alloy.